1. Field of the Invention
The present invention relates to zoom lens systems, and more particularly, though not exclusively, to a zoom lens system of a digital camera.
2. Description of the Related Art
Recently, imaging lenses for digital cameras and video cameras have been required to have smaller lens systems and to provide higher optical performances. In addition, there is a demand for digital cameras with reduced thicknesses to increase portability for the user.
To reduce the thickness of the camera, lens barrels generally have a collapsible structure that provides high storage space efficiency. A typical collapsible structure is provided with optical and mechanical systems for making it as thin as possible in the collapsed state.
It can have a relatively long time for the collapsible lens barrel to change from the collapsed (stored) state to a state in which shooting can be performed. In addition, a complex mechanism is required to store the lens barrel with high space efficiency.
Japanese Patent Laid-Open Nos. 2004-37967 and 2004-69808 discusses optical systems including reflective members that deflect an optical axis by approximately 90° to reduce the optical thickness in a direction toward the object (along the depth of the camera). In this way, the thickness of the camera can be reduced without using a collapsible structure.
On the other hand, a photoelectric converter of a typical solid-state image pickup device is placed at a relatively deep position (in a hole) with respect to an opening. Therefore, if the direction of incident light largely differs from a perpendicular direction, the light would be blocked by the opening, which leads to a reduction in sensitivity. Therefore, a typical optical shooting system using a conventional solid-state image pickup device is designed to be telecentric so that the angle of incidence on the image pickup device is close to perpendicular even in a peripheral region of an image plane.
In comparison, Japanese Patent Laid-Open No. 11-68074 (corresponding to U.S. Pat. No. 6,259,083) and Japanese Patent Laid-Open No. 2003-224249 discusses solid-state image pickup devices having improved hole structures wherein light can be efficiently received at the photoelectric conversion plane even when the direction of the incident light is inclined or varied.
A typical zoom lens used in a shooting system of a small digital camera is a retrofocus optical system including: a negative element (lens unit having a negative refractive power) at a position closest to an object; a first positive element (lens unit having a positive refractive power) on the image side of the negative element; and a second positive element (lens unit having a positive refractive power) at a position closest to an image plane.
During zooming, the first positive element is moved to change the magnification while compensating for the movement of the image plane using the negative element. In addition, the second positive element serves a refracting function for making the incident light on the image plane close to telecentric.
In the retrofocus optical system, to obtain a certain change in magnification with a small amount of movement, the system can use an element having a high positive refractive power as the first positive element.
However, to obtain a telecentric optical system, the second positive element must have a positive refractive power and be disposed separately from the first positive element. When the overall refractive power of the first and second positive elements is set to a certain refractive power, the refractive power of the first positive element can be reduced since the second positive element must have a portion of the overall positive refractive power. When the refractive power of the first positive element can be reduced, the first positive element can be moved a long distance during zooming, which increases the overall length of the lens system.
As described above, in a zoom lens including negative, positive, and positive elements, it is difficult to achieve both telecentricity and size reduction at the same time. When the solid-state image pickup devices discussed in the above-mentioned Japanese Patent Laid-Open Nos. 11-68074 and 2003-224249 are used, the optical system is not required to have high telecentricity. Therefore, there is a possibility that the structure including the negative, positive, and positive elements is not an optimum zoom lens structure.